Engines can be coupled to emission control devices, such as catalytic converters, to reduce exhaust emissions. However, these devices can become contaminated with sulfates, for example. In order to remove these contaminates, the temperature of the emission control device is raised significantly and a near stoichiometric air-fuel ratio is provided that alternates, or oscillates, around stoichiometry (between lean and rich).
Various methods can be used to raise emission control device temperature. One approach for raising temperature of a single exhaust path sequentially operates some cylinders lean, and then some rich to create heat. Such an approach is described in U.S. Pat. No. 5,974,788, for example. This approach tends to generate heat wherever oxidant storage is available, and in proportional to such storage. However, the inventors herein have recognized that under some conditions it is desirable to generate heat at locations other than simply were oxidant storage capacity happens to be located.
Another method operates some cylinder lean and others rich to provide coexisting oxidants and reductants in the exhaust gas that can react over the surface of a precious metal catalyst. Such an approach tends to generate heat as soon as the oxidants and reductants coexist over a catalyst surface. Such an approach is described in U.S. Pat. No. 6,189,316.
Again, the inventors herein have recognize that sometimes it is desired to generate heat in a location other than where the oxidants and reductants first co-exist over a catalyst.